Positron Annihilation Lifetime of Irradiated Polyimide

Ortho-positronium pick off annihilation lifetime gives the information of the microscopic free volume of polymers. Polyimide polymers such as Kapton are applied in many fields, but it was impossible to apply the positron lifetime method for free volume investigation because of no positronium formation. Here, we apply the idea of the free positron annihilation probability that is sum of the probability of annihilation by the positron and electrons on the molecular chain where the positron localized and that for the annihilation with the electrons on the neighboring molecular chains. The second term is probably affected by the free volume change. We have successfully shown the temperature dependence and the electron beam irradiation effect on the free volume change by observing the free positron annihilation lifetime for Kapton.

High Quality 3C-SiC Substrate for MOSFET Fabrication

Quantitative efficacies of several methods for stacking fault (SF) reduction are evaluated using Monte Carlo (MC) simulation. SF density on a 3C–SiC {001} surface depends on interactions of adjoining SFs: annihilation between counter pairs of SFs and termination by orthogonal SF pairs. However, SFs are not entirely eliminated when growth occurs on undulant-Si and switch back epitaxy (SBE) due to spontaneous SF collimation that suppresses the annihilation probability of counter SFs. The MC simulation also reveals the efficacy of SF reduction method which includes the growth of 3C–SiC on finite area bounded by side walls. One can theoretically reduce the SF density below 100 cm-1on 3C–SiC {001} surface. A practical way for eliminating the SF by termination at side walls is demonstrated, and it clearly exhibits that the SF density can be reduced under 120 cm-1.

Trinomial random walk with one or two imperfect absorbing barriers

Trinomial random walk, with one or two barriers, on the non-negative integers is discussed. At the barriers, the particle is either annihilated or reflects back to the system with respective probabilities 1 − ρ, ρ at the origin and 1 − ω, ω at L, 0 ≤ ρ,ω ≤ 1. Theoretical formulae are given for the probability distribution, its generating function as well as the mean of the time taken before absorption. In the one-boundary case, very qualitatively different asymptotic forms for the result, depending on the relationship between transition probabilities and the annihilation probability, are obtained.

SUBTHRESHOLD ANTIPROTON PRODUCTION IN HEAVY-ION COLLISIONS

We present a RQMD calculation of antiproton yields and their momentum distribution in Ne+NaF collisions at 2 GeV /u. The antiprotons can be produced below threshold due to multi-step excitations for which meson-baryon interactions play a considerable role. In this system the annihilation probability for an initially produced antiproton is predicted to be about 65%.

MOMENTUM DISTRIBUTION OF METALLIC ELECTRONS BY POSITRON ANNIHILATION

The angular correlation of photons from the two-photon decay of positrons has been measured for positrons annihilating in some 34 elements, mostly metals. These data give the momentum distribution of photons and hence of the center of mass of the annihilating electron–positron pairs. The momentum distributions are discussed in terms of the velocity dependence of the annihilation probability. It is concluded that the observed momentum distributions are primarily the momentum distributions of the conduction electrons in the metals. A higher momentum component is observed, which is attributed to ion core effects.